1. Field of the Invention
The present invention relates to the field of acousto-optic modulators and deflectors and, in particular, the use of an acoustic Fresnel zone plate to reduce the transducer power density.
2. Description of the Prior Art
The sound field profile generated in prior art acousto-optic modulators by the typical flat, rectangular transducer utilized therewith is not optimized for operation at high acousto carrier frequencies.
As noted in the article entitled "A Review of Acoustooptical Deflection and Modulation Devices", Proc. IEEE Vol. 54, No. 10, Oct. 1969, pgs. 1391-1401 by E. I. Gordon, power loading of the transducer and acoustic medium can be avoided by operating in the far field of the acoustic beam, that is, by placing the nominal beam focus or waist outside the transducer. However, with the flat transducer, the beam waist occurs at the transducer.
The nominal beam focus or waist can be placed outside the acoustic medium by the use of cylindrical transducers, which produce cylindrically focused beams, or by the use of acoustical mirrors. An article by M. G. Cohen et al, "Focusing of Microwave Acoustic Beams", Journal of Applied Physics, Volume 38, No. 5, April, 1967, pgs. 2340-2344, describes techniques for focusing a sound field which include deposition of a thin film transducer on a curved surface and the use of an acoustical mirror. Neither technique is satisfactory because thin film transducers have notoriously low efficiency and curved surfaces require more labor to fabricate than flat surfaces.
The power density at the transducer of an acousto-optic device has been determined to be proportional to the fourth power of the acoustic carrier frequency if the transducer is flat and a single element. Since high performance requires high carrier frequencies, extremely high power densities are required in the transducer. The power density at the transducer can be reduced by providing an array of flat transducers. However, it is necessary to focus the acoustically wide, low intensity beam into a narrow, high intensity beam.
Y. Ohta et al, in an article entitled "Improved A-O Modulator for Wideband Laser Recording System", Electro-Optical Systems Design, February, 1979, pgs. 26-29, discloses an acousto-optic modulator having a single flat rectangular transducer bonded by indium, five electrodes on the transducer upper face having a predetermined length and interelectrode spacing. Each transducer element is driven by an electric field having predetermined relative amplitudes. The amplitude and phase distribution of the acoustic wave at the transducer produced by the Ohta et al device results in the broadening of the diffracted light intensity distribution which is necessary for high performance scanning applications.